Method of improving the corrosion resistance of a metal substrate

ABSTRACT

The invention provides a method of improving the corrosion resistance of a metal substrate. The method comprises:
     (a) electrophoretically depositing on the substrate a curable electrodepositable coating composition to form a coating over at least a portion of the substrate, and   (b) heating the substrate to a temperature and for a time sufficient to cure the coating on the substrate. The electrodepositable coating composition comprises a resinous phase dispersed in an aqueous medium, the resinous phase comprising:
       (1) an ungelled active hydrogen-containing, cationic salt group-containing resin electrodepositable on a cathode;   (2) an at least partially blocked polyisocyanate curing agent; and   (3) a pigment component comprising an inorganic, platelike pigment having an average equivalent spherical diameter of at least 0.2 microns. The electrodepositable coating composition demonstrates a pigment-to-binder ratio of at least 0.5. The coating composition contains less than 8 percent by weight of a grind vehicle.

FIELD OF THE INVENTION

The present invention is directed to a method of improving the corrosionresistance of a metal substrate.

BACKGROUND OF THE INVENTION

Electrodeposition as a coating application method involves deposition ofa film-forming composition onto a conductive substrate under theinfluence of an applied electrical potential. Electrodeposition hasbecome standard in the coatings industry because, by comparison withnon-electrophoretic coating means, electrodeposition offers increasedpaint utilization with less waste, improved corrosion protection to thesubstrate, and minimal environmental contamination.

Initially, electrodeposition was conducted with the workpiece to becoated serving as the anode. This was familiarly referred to as anionicelectrodeposition. However, in 1972 cationic electrodeposition wasintroduced commercially and has become an industry standard. Today,cationic electrodeposition is by far the prevalent method ofelectrodeposition. In fact, a cationic primer coating is applied byelectrodeposition to more than 80 percent of all motor vehicles producedthroughout the world.

Electrodepositable coatings typically contain pigments that servenumerous purposes. Usually, the pigments are introduced into thecoatings after incorporation into a grinding vehicle by a millingprocess. The use of a grinding vehicle reduces pigment agglomeration andallows for homogeneous dispersion of the pigment into the coating bulk,but it involves additional formulation steps at a higher cost. It alsomakes it difficult to incorporate a high level of pigment into thecoating and weakens the corrosion barrier properties of the coating bylowering the pigment-to-binder (P:B) ratio and crosslink density of thecoating.

There remains a need in the coatings industry for a cost effectiveelectrodepositable primer composition which allows for higher pigmentloadings, to provide improved corrosion resistance to a metal substrate.

SUMMARY OF THE INVENTION

The present invention is directed to a method of improving the corrosionresistance of a metal substrate. The method comprises:

(a) electrophoretically depositing on the substrate a curableelectrodepositable coating composition to form an electrodepositedcoating over at least a portion of the substrate, and

(b) heating the substrate to a temperature and for a time sufficient tocure the electrodeposited coating on the substrate. The curableelectrodepositable coating composition comprises a resinous phasedispersed in an aqueous medium, the resinous phase comprising:

(1) an ungelled active hydrogen-containing, cationic saltgroup-containing resin electrodepositable on a cathode;

(2) an at least partially blocked polyisocyanate curing agent; and

(3) a pigment component. The pigment component comprises an inorganic,platelike pigment having an average equivalent spherical diameter of atleast 0.2 microns, and the inorganic platelike pigment is present in theresinous phase in an amount such that the electrodepositable coatingcomposition demonstrates a pigment-to-binder ratio of at least 0.5. Theelectrodepositable coating composition contains less than 8 percent byweight of a grind vehicle, based on the total weight of solids in theelectrodepositable coating composition. By improvement is meant that,after coating with the curable electrodepositable coating compositionand after curing as described above, the metal substrate demonstratesimproved salt spray corrosion resistance compared to a metal substrateof the same material that has been coated with a curableelectrodepositable coating composition comprising a resinous phasedispersed in an aqueous medium, where the resinous phase contains: (1)an ungelled active hydrogen-containing, cationic salt group-containingresin electrodepositable on a cathode and (2) an at least partiallyblocked polyisocyanate curing agent as above, but does not contain thepigment component (3).

DETAILED DESCRIPTION OF THE INVENTION

Other than in the operating examples, or where otherwise indicated, allnumbers expressing quantities of ingredients, reaction conditions and soforth used in the specification and claims are to be understood as beingmodified in all instances by the term “about.” Accordingly, unlessindicated to the contrary, the numerical parameters set forth in thefollowing specification and attached claims are approximations that mayvary depending upon the desired properties sought to be obtained by thepresent invention. At the very least, and not as an attempt to limit theapplication of the doctrine of equivalents to the scope of the claims,each numerical parameter should at least be construed in light of thenumber of reported significant digits and by applying ordinary roundingtechniques.

Notwithstanding that the numerical ranges and parameters setting forththe broad scope of the invention are approximations, the numericalvalues set forth in the specific examples are reported as precisely aspossible. Any numerical values, however, inherently contain certainerrors necessarily resulting from the standard deviation found in theirrespective testing measurements.

Also, it should be understood that any numerical range recited herein isintended to include all sub-ranges subsumed therein. For example, arange of 1″ to 10″ is intended to include all sub-ranges between andincluding the recited minimum value of 1 and the recited maximum valueof 10, that is, having a minimum value equal to or greater than 1 and amaximum value of equal to or less than 10.

As used in this specification and the appended claims, the articles “a,”“an,” and “the” include plural referents unless expressly andunequivocally limited to one referent.

The various examples of the present invention as presented herein areeach understood to be non-limiting with respect to the scope of theinvention.

In the method of the present invention, the curable electrodepositablecoating composition can be electrophoretically deposited onto at least aportion of any of a variety of metal substrates. Suitable metalsubstrates can include ferrous metals and non-ferrous metals. Suitableferrous metals include iron, steel, and alloys thereof. Non-limitingexamples of useful steel materials include cold-rolled steel, galvanized(i.e., zinc coated) steel, electrogalvanized steel, stainless steel,pickled steel, zinc-aluminum alloys coated upon steel such as thoseavailable under the names GALVANNEAL®, GALVALUME®, AND GALVAN®, andcombinations thereof. Useful non-ferrous metals include conductivecarbon coated materials, aluminum, copper, zinc, magnesium and alloysthereof. Cold rolled steel also is suitable when pretreated with asolution such as a metal phosphate solution, an aqueous solutioncontaining at least one Group IIIB or IVB metal, an organophosphatesolution, an organophosphonate solution and combinations of the above asare discussed below. Combinations or composites of ferrous andnon-ferrous metals can also be used.

The curable electrodepositable coating compositions can be applied toeither bare metal or pretreated metal substrates. By “bare metal” ismeant a virgin metal substrate that has not been treated with apretreatment composition such as conventional phosphating solutions,heavy metal rinses and the like. Additionally, for purposes of thepresent invention, ‘bare metal’ substrates can include a cut edge of asubstrate that is otherwise treated and/or coated over the non-edgesurfaces of the substrate.

Before any treatment or application of any coating composition, thesubstrate optionally may be formed into an object of manufacture. Acombination of more than one metal substrate of the same or differentmaterials can be assembled together to form such an object ofmanufacture.

Also, it should be understood that as used herein, an electrodepositablecomposition or coating formed “over” at least a portion of a “substrate”refers to a composition formed directly on at least a portion of thesubstrate surface, as well as a composition or coating formed over anycoating or pretreatment material which was previously applied to atleast a portion of the substrate.

That is, the “substrate” upon which the coating composition iselectrodeposited can comprise any electroconductive substrates includingthose described above to which one or more pretreatment and/or primercoatings have been previously applied. For example, the “substrate” cancomprise a metallic substrate and a weldable primer coating over atleast a portion of the substrate surface. The electrodepositable coatingcomposition described above is then electrodeposited and cured over atleast a portion thereof. One or more top coating compositions asdescribed in detail below are subsequently applied over at least aportion of the cured electrodeposited coating.

For example, the substrate can comprise any of the foregoingelectroconductive substrates and a pre-treatment composition appliedover at least a portion of the substrate, the pretreatment compositioncomprising a solution that contains one or more Group IIIB or IVBelement-containing compounds, or mixtures thereof, solubilized ordispersed in a carrier medium, typically an aqueous medium. The GroupIIIB and IVB elements are defined by the CAS Periodic Table of theElements as shown, for example, in the Handbook of Chemistry andPhysics, (60th Ed. 1980). Transition metal compounds and rare earthmetal compounds typically are compounds of zirconium, titanium, hafnium,yttrium and cerium and mixtures thereof. Typical zirconium compounds maybe selected from hexafluorozirconic acid, alkali metal and ammoniumsalts thereof, ammonium zirconium carbonate, zirconyl nitrate, zirconiumcarboxylates and zirconium hydroxy carboxylates such ashydrofluorozirconic acid, zirconium acetate, zirconium oxalate, ammoniumzirconium glycolate, ammonium zirconium lactate, ammonium zirconiumcitrate, and mixtures thereof.

The pretreatment composition carrier also can contain a film-formingresin, for example, the reaction products of one or more alkanolaminesand an epoxy-functional material containing at least two epoxy groups,such as those disclosed in U.S. Pat. No. 5,653,823. Other suitableresins include water soluble and water dispersible polyacrylic acidssuch as those as disclosed in U.S. Pat. Nos. 3,912,548 and 5,328,525;phenol-formaldehyde resins as described in U.S. Pat. No. 5,662,746;water soluble polyamides such as those disclosed in WO 95/33869;copolymers of maleic or acrylic acid with allyl ether as described inCanadian patent application 2,087,352; and water soluble and dispersibleresins including epoxy resins, aminoplasts, phenol-formaldehyde resins,tannins, and polyvinyl phenols as discussed in U.S. Pat. No. 5,449,415.

Further, non-ferrous or ferrous substrates can be pretreated with anon-insulating layer of organophosphates or organophosphonates such asthose described in U.S. Pat. Nos. 5,294,265 and 5,306,526. Suchorganophosphate or organophosphonate pretreatments are availablecommercially from PPG Industries, Inc. under the trade name NUPAL®.Application to the substrate of a non-conductive coating, such as NUPAL,typically is followed by the step of rinsing the substrate withdeionized water prior to the coalescing of the coating. This ensuresthat the layer of the non-conductive coating is sufficiently thin to benon-insulating, i.e., sufficiently thin such that the non-conductivecoating does not interfere with electroconductivity of the substrate,allowing subsequent electrodeposition of a electrodepositable coatingcomposition. The pretreatment coating composition can further comprisesurfactants that function as aids to improve wetting of the substrate.Generally, the surfactant materials are present in an amount of lessthan about 2 weight percent on a basis of total weight of thepretreatment coating composition. Other optional materials in thecarrier medium include defoamers.

Due to environmental concerns, the pretreatment coating composition canbe free of chromium-containing materials, i.e., the composition containsless than about 2 weight percent of chromium-containing materials(expressed as CrO₃), typically less than about 0.05 weight percent ofchromium-containing materials, based on the total weight of thepretreatment composition.

In a typical pre-treatment process, before depositing the pre-treatmentcomposition upon the surface of the metal substrate, it is usualpractice to remove foreign matter from the metal surface by thoroughlycleaning and degreasing the surface. The surface of the metal substratecan be cleaned by physical or chemical means, such as by mechanicallyabrading the surface or cleaning/degreasing with commercially availablealkaline or acidic cleaning agents which are well known to those skilledin the art, such as sodium metasilicate and sodium hydroxide. Anon-limiting example of a suitable cleaning agent is CHEMKLEEN® 163, analkaline-based cleaner commercially available from PPG Pretreatment andSpecialty Products of Troy, Mich. Acidic cleaners also can be used.Following the cleaning step, the metal substrate is usually rinsed withwater in order to remove any residue. The metal substrate can beair-dried using an air knife, by flashing off the water by briefexposure of the substrate to a high temperature or by passing thesubstrate between squeegee rolls. The pretreatment coating compositioncan be deposited upon at least a portion of the outer surface of themetal substrate. Usually, the entire outer surface of the metalsubstrate is treated with the pretreatment composition. The thickness ofthe pretreatment film can vary, but is generally less than about 1micrometer, usually ranges from about 1 to about 500 nanometers, andmore often ranges from about 10 to about 300 nanometers.

The pretreatment coating composition is applied to the surface of themetal substrate by any conventional application technique, such as byspraying, immersion or roll coating in a batch or continuous process.The temperature of the pretreatment coating composition at applicationis typically about 10° C. to about 85° C., and often about 15° C. toabout 60° C. The pH of the pretreatment coating composition atapplication generally ranges from 2.0 to 5.5, and typically from 3.5 to5.5. The pH of the medium may be adjusted using mineral acids such ashydrofluoric acid, fluoroboric acid, phosphoric acid, sulfamic acid, andthe like, including mixtures thereof; organic acids such as lactic acid,acetic acid, citric acid, or mixtures thereof; and water soluble orwater dispersible bases such as sodium hydroxide, ammonium hydroxide,ammonia, or amines such as triethylamine, methylethyl amine, or mixturesthereof.

Continuous processes typically are used in the coil coating industry andalso for mill application. The pretreatment coating composition can beapplied by any of these conventional processes. For example, in the coilindustry, the substrate typically is cleaned and rinsed and thencontacted with the pretreatment coating composition by roll coating witha chemical coater. The treated strip is then dried by heating, paintedand baked by conventional coil coating processes.

Mill application of the pretreatment composition can be by immersion,spray or roll coating applied to the freshly mill-manufactured metalstrip. Excess pretreatment composition is typically removed by wringerrolls. After the pretreatment composition has been applied to the metalsurface, the metal can be rinsed with deionized water and dried at roomtemperature or at elevated temperatures to remove excess moisture fromthe treated substrate surface and cure any curable coating components toform the pretreatment coating. Alternatively, the treated substrate canbe heated to a temperature ranging from 65° C. to 125° C. for 2 to 30seconds to produce a coated substrate having a dried residue of thepretreatment coating composition thereon. If the substrate is alreadyheated from the hot melt production process, no post application heatingof the treated substrate is required to facilitate drying. Thetemperature and time for drying the coating will depend upon suchvariables as the percentage of solids in the coating, components of thecoating composition and type of substrate.

The film coverage of the residue of the pretreatment compositiongenerally ranges from 1 to 10,000 milligrams per square meter (mg/m2),and usually from 10 to 400 mg/m2.

A layer of a weldable primer also can be applied to the substrate,whether or not the substrate has been pretreated. A typical weldableprimer is a zinc-rich mill applied organic film-forming composition,which is commercially available from PPG, Pittsburgh, Pa. as BONAZINC®This weldable primer can be applied to a thickness of at least 1micrometer and typically to a thickness of 3 to 4 micrometers. Otherweldable primers, such as iron phosphide-rich primers, are commerciallyavailable.

The electrodeposition process of the present invention typicallyinvolves immersing the electroconductive substrate into anelectrodeposition bath of an aqueous electrodepositable composition; thesubstrate, which is usually metal, serving as a cathode in an electricalcircuit comprising the cathode and an anode. Sufficient electricalcurrent is applied between the electrodes to deposit a substantiallycontinuous, adherent film of the electrodepositable coating compositiononto at least a portion of the surface of the electroconductivesubstrate. Electrodeposition is usually carried out at a constantvoltage in the range of from 1 volt to several thousand volts, typicallybetween 50 and 500 volts. Current density is usually between 1.0 ampereand 15 amperes per square foot (10.8 to 161.5 amperes per square meter)and tends to decrease quickly during the electrodeposition process,indicating formation of a continuous, self-insulating film.

The electrodepositable coating composition used in the method of thepresent invention comprises a resinous phase dispersed in an aqueousmedium. The resinous phase comprises (1) one or more ungelled, activehydrogen-containing, cationic salt group-containing resins (i.e.,polymers), typically active hydrogen group-containing, cationic aminesalt group-containing polymers, which are electrodepositable on acathode; (2) one or more at least partially blocked polyisocyanatecuring agents; and (3) a pigment component.

The term “curable”, as used for example in connection with a curablecomposition, means that the indicated composition is polymerizable orcross linkable through functional groups, e.g., by means that include,but are not limited to, thermal (including ambient cure) and/orcatalytic exposure.

The term “cure”, “cured” or similar terms, as used in connection with acured or curable composition, e.g., a “cured composition” of somespecific description, means that at least a portion of the polymerizableand/or crosslinkable components that form the curable composition ispolymerized and/or crosslinked. Additionally, curing of a compositionrefers to subjecting said composition to curing conditions such as butnot limited to thermal curing, leading to the reaction of the reactivefunctional groups of the composition, and resulting in polymerizationand formation of a polymerizate. When a polymerizable composition issubjected to curing conditions, following polymerization and afterreaction of most of the reactive end groups occurs, the rate of reactionof the remaining unreacted reactive end groups becomes progressivelyslower. The polymerizable composition can be subjected to curingconditions until it is at least partially cured. The term “at leastpartially cured” means that upon subjecting the composition to curingconditions, reaction of at least a portion of the reactive groups of thecomposition occurs, to form a polymerizate.

As used herein, “substantially uncured” means that the coatingcomposition, after application to the surface of a substrate, forms afilm which is substantially uncrosslinked; i.e., it is not heated to atemperature sufficient to induce significant crosslinking and there issubstantially no chemical reaction between the polymeric component andthe curing agent.

The term “reactive” refers to a functional group capable of undergoing achemical reaction with itself and/or other functional groupsspontaneously or upon the application of heat or in the presence of acatalyst or by any other means known to those skilled in the art.

Examples of ungelled active hydrogen-containing, cationic saltgroup-containing resins that are suitable for use in theelectrodepositable coating compositions, typically as the mainfilm-forming polymer, can include any of a number of cationic polymerswell known in the art so long as the polymers are “water dispersible,”i.e., adapted to be solubilized, dispersed or emulsified in water. Suchpolymers comprise cationic functional groups to impart a positivecharge.

By “ungelled” is meant the resins are substantially free of crosslinkingand demonstrate a measurable intrinsic viscosity when dissolved in asuitable solvent, as determined, for example, in accordance withASTM-D1795 (published 2013) or ASTM-D4243 (published 2016). Theintrinsic viscosity of the reaction product is an indication of itsmolecular weight. As used herein, a reaction product that is“substantially free of crosslinking” refers to a reaction product thathas a weight average molecular weight (Mw), as determined by gelpermeation chromatography, of less than 1,000,000 Da.

Also, as used herein, the term “polymer” is meant to refer to oligomersand both homopolymers and copolymers, and is used interchangeably with“resin”. Unless stated otherwise, as used in the specification and theclaims, molecular weights are number average molecular weights forpolymeric materials indicated as “M_(n)” and obtained by gel permeationchromatography using polystyrene standards in an art-recognized manner.

Suitable examples of such cationic film-forming resins can includeactive hydrogen-containing, cationic polymers derived from one or moreof a polyepoxide polymer, an acrylic polymer, a polyurethane polymer, apolyester polymer, mixtures thereof, and copolymers thereof; for examplea polyester-polyurethane polymer. Typically, the resin (1) comprises anactive hydrogen-containing, cationic polymer derived from a polyepoxidepolymer and/or an acrylic polymer. Note that the phrase “and/or” whenused in a list is meant to encompass alternative embodiments includingeach individual component in the list as well as any combination ofcomponents. For example, the list “A, B, and/or C” is meant to encompassseven separate embodiments that include A, or B, or C, or A+B, or A+C,or B+C, or A+B+C.

As aforementioned, the polymers which are suitable for use as thecationic resin (1), comprise active hydrogens as curing reaction sites.The term “active hydrogen” refers to those groups which are reactivewith isocyanates as determined by the Zerewitnoff test as is describedin the JOURNAL OF THE AMERICAN CHEMICAL SOCIETY, Vol. 49, page 3181(1927). In one example of the present invention, the active hydrogensare derived from hydroxyl groups, primary amine groups and/or secondaryamine groups.

Suitable polyepoxides polymers for use as the activehydrogen-containing, cationic salt group-containing resin include, forexample, a polyepoxide chain-extended by reacting together a polyepoxideand a polyhydroxyl group-containing material such as alcoholic hydroxylgroup-containing materials and phenolic hydroxyl group-containingmaterials to chain extend or increase the molecular weight of thepolyepoxide.

A chain-extended polyepoxide is typically prepared by reacting togetherthe polyepoxide and polyhydroxyl group-containing material neat or inthe presence of an inert organic solvent such as a ketone, includingmethyl isobutyl ketone and methyl amyl ketone, aromatics such as tolueneand xylene, and glycol ethers such as the dimethyl ether of diethyleneglycol. The reaction is usually conducted at a temperature of about 80°C. to 160° C. for about 30 to 180 minutes until an epoxygroup-containing resinous reaction product is obtained.

The equivalent ratio of reactants, i.e., epoxy:polyhydroxylgroup-containing material, is typically from about 1.00:0.75 to1.00:2.00.

In general the epoxide equivalent weight of the polyepoxide will rangefrom 100 to about 2000, typically from about 180 to 500, prior to chainextension. The epoxy compounds may be saturated or unsaturated, cyclicor acyclic, aliphatic, alicyclic, aromatic or heterocyclic. They maycontain substituents such as halogen, hydroxyl, and ether groups.

Examples of polyepoxides are those having a 1,2-epoxy equivalencygreater than one and usually about two; that is, polyepoxides which haveon average two epoxide groups per molecule. The most commonly usedpolyepoxides are polyglycidyl ethers of cyclic polyols, for example,polyglycidyl ethers of polyhydric phenols such as Bisphenol A,resorcinol, hydroquinone, benzenedimethanol, phloroglucinol, andcatechol; or polyglycidyl ethers of polyhydric alcohols such asalicyclic polyols, particularly cycloaliphatic polyols such as1,2-cyclohexane diol, 1,4-cyclohexane diol,2,2-bis(4-hydroxycyclohexyl)propane, 1,1-bis(4-hydroxycyclohexyl)ethane,2-methyl-1,1-bis(4-hydroxycyclohexyl)propane,2,2-bis(4-hydroxy-3-tertiarybutylcyclohexyl)propane,1,3-bis(hydroxymethyl)cyclohexane and 1,2-bis(hydroxymethyl)cyclohexane.Examples of aliphatic polyols include, inter alia, trimethylpentanedioland neopentyl glycol.

Polyhydroxyl group-containing materials used to chain extend or increasethe molecular weight of the polyepoxide may additionally be polymericpolyols.

Suitable acrylic polymers that may be used to prepare the activehydrogen-containing, cationic salt group-containing resin includecopolymers of one or more alkyl esters of acrylic acid or methacrylicacid optionally together with one or more other polymerizableethylenically unsaturated monomers. Suitable alkyl esters of acrylicacid or methacrylic acid include methyl methacrylate, ethylmethacrylate, butyl methacrylate, ethyl acrylate, butyl acrylate, and2-ethyl hexyl acrylate. Suitable other copolymerizable ethylenicallyunsaturated monomers include nitriles such acrylonitrile andmethacrylonitrile, vinyl and vinylidene halides such as vinyl chlorideand vinylidene fluoride and vinyl esters such as vinyl acetate. Acid andanhydride functional ethylenically unsaturated monomers such as acrylicacid, methacrylic acid or anhydride, itaconic acid, maleic acid oranhydride, or fumaric acid may be used. Amide functional monomersincluding, acrylamide, methacrylamide, and N-alkyl substituted(meth)acrylamides are also suitable. Vinyl aromatic compounds such asstyrene and vinyl toluene are also suitable.

Functional groups such as hydroxyl and amino groups may be incorporatedinto the acrylic polymer by using functional monomers such ashydroxyalkyl acrylates and methacrylates or aminoalkyl acrylates andmethacrylates. Tertiary amino groups (for conversion to cationic saltgroups) may be incorporated into the acrylic polymer by usingdialkylaminoalkyl (meth)acrylate functional monomers such asdimethylaminoethyl methacrylate, diethylaminoethyl methacrylate,dipropylaminoethyl methacrylate, and the like.

Epoxide functional groups (for conversion to cationic salt groups) maybe incorporated into the acrylic polymer by using functional monomerssuch as glycidyl acrylate and methacrylate,3,4-epoxycyclohexylmethyl(meth)acrylate, allyl glycidyl ether, or2-(3,4-epoxycyclohexyl)ethyl(meth)acrylate. Alternatively, epoxidefunctional groups may be incorporated into the acrylic polymer byreacting hydroxyl groups on the acrylic polymer with an epihalohydrin ordihalohydrin such as epichlorohydrin or dichlorohydrin in the presenceof alkali.

The acrylic polymer may be prepared by traditional free radicalinitiated polymerization techniques, such as solution or emulsionpolymerization, as known in the art using suitable catalysts whichinclude organic peroxides and azo type compounds and optionally chaintransfer agents such as alpha-methyl styrene dimer and tertiary dodecylmercaptan.

The active hydrogen-containing, cationic salt group-containing resin mayalternatively or additionally be prepared from a polyester. Thepolyesters may be prepared in a known manner by condensation ofpolyhydric alcohols and polycarboxylic acids. Suitable polyhydricalcohols include, for example, ethylene glycol, propylene glycol,butylene glycol, 1,6-hexylene glycol, neopentyl glycol, diethyleneglycol, glycerol, trimethylol propane, and pentaerythritol.

Examples of suitable polycarboxylic acids used to prepare the polyesterinclude succinic acid, adipic acid, azelaic acid, sebacic acid, maleicacid, fumaric acid, phthalic acid, tetrahydrophthalic acid,hexahydrophthalic acid, and trimellitic acid. Besides the polycarboxylicacids mentioned above, functional equivalents of the acids such asanhydrides where they exist or lower alkyl esters of the acids such asthe methyl esters may be used.

The polyesters contain a portion of free hydroxyl groups (done by usingexcess polyhydric alcohol and/or higher polyols during preparation ofthe polyester) which are available for crosslinking reactions.

Epoxide functional groups may be incorporated into the polyester byreacting hydroxyl groups on the polyester with an epihalohydrin ordihalohydrin such as epichlorohydrin or dichlorohydrin in the presenceof alkali.

Alkanolamines and dialkanolamines may be used in combination with thepolyols in the preparation of the polyester, and the amine groups maylater be alkylated to form tertiary amino groups for conversion tocationic salt groups. Likewise, tertiary amines such asN,N-dialkylalkanolamines and N-alkyldialkanolamines may be used in thepreparation of the polyester. Examples of suitable tertiary aminesinclude those N-alkyl dialkanolamines disclosed in U.S. Pat. No.5,483,012, at column 3, lines 49-63. Suitable polyesters for use in theprocess of the present invention include those disclosed in U.S. Pat.No. 3,928,157.

Polyurethanes can also be used as the active hydrogen-containing,cationic salt group-containing resin. Among the polyurethanes which canbe used are polymeric polyols which are prepared by reacting polyesterpolyols or acrylic polyols such as those mentioned above with apolyisocyanate such that the OH/NCO equivalent ratio is greater than 1:1so that free hydroxyl groups are present in the product. Smallerpolyhydric alcohols such as those disclosed above for use in thepreparation of the polyester may also be used in place of or incombination with the polymeric polyols.

The organic polyisocyanate used to prepare the polyurethane polymer isoften an aliphatic polyisocyanate. Diisocyanates and/or higherpolyisocyanates are suitable.

Examples of suitable aliphatic diisocyanates are straight chainaliphatic diisocyanates such as 1,4-tetramethylene diisocyanate and1,6-hexamethylene diisocyanate. Also, cycloaliphatic diisocyanates canbe employed. Examples include isophorone diisocyanate and4,4′-methylene-bis-(cyclohexyl isocyanate). Examples of suitable aralkyldiisocyanates are meta-xylylene diisocyanate andα,α,α′,α′-tetramethylmeta-xylylene diisocyanate.

Isocyanate prepolymers, for example, reaction products ofpolyisocyanates with polyols such as neopentyl glycol and trimethylolpropane or with polymeric polyols such as polycaprolactone diols andtriols (NCO/OH equivalent ratio greater than one) can also be used inthe preparation of the polyurethane.

Hydroxyl functional tertiary amines such as N,N-dialkylalkanolamines andN-alkyl dialkanolamines may be used in combination with the otherpolyols in the preparation of the polyurethane. Examples of suitabletertiary amines include those N-alkyl dialkanolamines disclosed in U.S.Pat. No. 5,483,012, at column 3, lines 49-63.

Epoxide functional groups may be incorporated into the polyurethane byreacting hydroxyl groups on the polyurethane with an epihalohydrin ordihalohydrin such as epichlorohydrin or dichlorohydrin in the presenceof alkali.

The cationic resin used in the electrodepositable composition containscationic salt groups. The cationic salt groups may be incorporated intothe resin by any means known in the art depending on the type of resinand/or active hydrogen group, such as by acidifying tertiary aminegroups in the resin as described below or by reacting epoxide groups inthe resin with a cationic salt group former. By “cationic salt groupformer” is meant a material which is reactive with epoxy groups andwhich can be acidified before, during, or after reaction with epoxygroups to form cationic salt groups. Examples of suitable materialsinclude amines such as primary or secondary amines which can beacidified after reaction with the epoxy groups to form amine saltgroups, or tertiary amines which can be acidified prior to reaction withthe epoxy groups and which after reaction with the epoxy groups formquaternary ammonium salt groups. Examples of other cationic salt groupformers are sulfides that can be mixed with acid prior to reaction withthe epoxy groups and form ternary sulfonium salt groups upon subsequentreaction with the epoxy groups.

When amines are used as the cationic salt formers, monoamines are oftenused, and hydroxyl-containing amines are particularly suitable.Polyamines may be used but are not recommended because of a tendency togel the resin.

In a typical example of the invention, the cationic saltgroup-containing resin contains amine salt groups, which are derivedfrom an amine containing a nitrogen atom to which is bonded at leastone, usually two, alkyl groups having a hetero atom in a beta-positionrelative to the nitrogen atom. A hetero atom is a non-carbon ornon-hydrogen atom, typically oxygen, nitrogen, or sulfur.

Hydroxyl-containing amines, when used as the cationic salt groupformers, may impart the resin with amine groups comprising a nitrogenatom to which is bonded at least one alkyl group having a hetero atom ina beta-position relative to the nitrogen atom. Examples ofhydroxyl-containing amines are alkanolamines, dialkanolamines, alkylalkanolamines, and aralkyl alkanolamines containing from 1 to 18 carbonatoms, usually 1 to 6 carbon atoms in each of the alkanol, alkyl andaryl groups. Specific examples include ethanolamine,N-methylethanolamine, diethanolamine, N-phenylethanolamine,N,N-dimethylethanolamine, N-methyldiethanolamine, triethanolamine andN-(2-hydroxyethyl)-piperazine.

Minor amounts of amines such as mono, di, and trialkylamines and mixedaryl-alkyl amines which do not contain hydroxyl groups, or aminessubstituted with groups other than hydroxyl which do not negativelyaffect the reaction between the amine and the epoxy may also be used,but their use is not preferred. Specific examples include ethylamine,methylethylamine, triethylamine, N-benzyldimethylamine, dicocoamine andN, N-dimethylcyclohexylamine.

The reaction of a primary and/or secondary amine with epoxide groups onthe polymer takes place upon mixing of the amine and polymer. The aminemay be added to the polymer or vice versa. The reaction can be conductedneat or in the presence of a suitable solvent such as methyl isobutylketone, xylene, or 1-methoxy-2-propanol. The reaction is generallyexothermic and cooling may be desired. However, heating to a moderatetemperature of about 50 to 150° C. may be done to hasten the reaction.

The tertiary amine functional polymer (or the reaction product of theprimary and/or secondary amine and the epoxide functional polymer) isrendered cationic and water dispersible by at least partialneutralization with an acid. Suitable acids include organic andinorganic acids such as formic acid, acetic acid, lactic acid,phosphoric acid, dimethylolpropionic acid, and sulfamic acid. Lacticacid is used most often. The extent of neutralization varies with theparticular reaction product involved. However, sufficient acid should beused to disperse the electrodepositable composition in water. Typically,the amount of acid used provides at least 20 percent of all of the totalneutralization. Excess acid may also be used beyond the amount requiredfor 100 percent total neutralization.

In the reaction of a tertiary amine with an epoxide functional polymer,the tertiary amine can be pre-reacted with the neutralizing acid to formthe amine salt and then the amine salt reacted with the polymer to forma quaternary salt group-containing resin. The reaction is conducted bymixing the amine salt with the polymer in water. Typically the water ispresent in an amount ranging from about 1.75 to about 20 percent byweight based on total reaction mixture solids.

In forming the quaternary ammonium salt group-containing resin, thereaction temperature can be varied from the lowest temperature at whichthe reaction will proceed, generally at or slightly above roomtemperature, to a maximum temperature of about 100° C. (at atmosphericpressure). At higher pressures, higher reaction temperatures may beused. Usually the reaction temperature is in the range of about 60 to100° C. Solvents such as a sterically hindered ester, ether, orsterically hindered ketone may be used, but their use is not necessary.

In addition to or in lieu of the primary, secondary, and/or tertiaryamines disclosed above, a portion of the amine that is reacted with thepolymer can be a ketimine of a polyamine, such as is described in U.S.Pat. No. 4,104,147, column 6, line 23 to column 7, line 23. The ketiminegroups decompose upon dispersing the amine-epoxy reaction product inwater.

In addition to resins containing amine salts and quaternary ammoniumsalt groups, cationic resins containing ternary sulfonium groups may beused in forming the cationic salt group-containing resin. Examples ofthese resins and their method of preparation are described in U.S. Pat.No. 3,793,278 to DeBona and U.S. Pat. No. 3,959,106 to Bosso et al.

The extent of ionic salt group formation should be such that when theresin is mixed with an aqueous medium and the other ingredients, astable dispersion of the electrodepositable composition will form. By“stable dispersion” is meant one that does not settle or is easilyredispersible if some settling occurs. Moreover, the dispersion shouldbe of sufficient ionic character that the dispersed particles willmigrate toward and electrodeposit on a cathode or anode, as appropriate,when an electrical potential is set up between an anode and a cathodeimmersed in the aqueous dispersion.

Generally, the cationic resin is ungelled as defined above and containsfrom about 0.1 to 3.0, often from about 0.1 to 0.7 millequivalents ofcationic salt group per gram of resin solids.

The active hydrogens associated with the cationic polymer include anyactive hydrogens which are reactive with isocyanates within thetemperature range of about 93° C. to 204° C., usually about 121° C. to177° C. Typically, the active hydrogens are selected from the groupconsisting of hydroxyl and primary and secondary amino, including mixedgroups such as hydroxyl and primary amino. Often, the polymer will havean active hydrogen content of about 1.7 to 10 millequivalents, moreoften about 2.0 to 5 millequivalents of active hydrogen per gram ofpolymer solids.

The cationic salt group-containing resin (1) can be present in theelectrodepositable composition used in the processes of the presentinvention in an amount ranging from 20 to 80 percent, often from 30 to75 percent by weight, and typically from 50 to 70 percent by weightbased on the total combined weight of the cationic salt group-containingresin and the curing agent.

The polyisocyanate curing agent (2) used in the curableelectrodepositable coating composition is at least partially blocked.Often the polyisocyanate curing agent is a fully blocked polyisocyanatewith substantially no free isocyanate groups. The polyisocyanate can bean aliphatic or an aromatic polyisocyanate or a mixture of the two.Diisocyanates are used most often, although higher polyisocyanates canbe used in place of or in combination with diisocyanates.

Examples of polyisocyanates suitable for use as curing agents includeall those disclosed above as suitable for use in the preparation of thepolyurethane. In a particular example, the polyisocyanate is isophoronediisocyanate blocked with trimethylol propane and/or methyl ethylketoxime.

Any suitable aliphatic, cycloaliphatic, or aromatic alkyl monoalcohol orphenolic compound may be used as a capping (blocking) agent for thepolyisocyanate including, for example, lower aliphatic alcohols such asmethanol, ethanol, and n-butanol; cycloaliphatic alcohols such ascyclohexanol; aromatic-alkyl alcohols such as phenyl carbinol andmethylphenyl carbinol; and phenolic compounds such as phenol itself andsubstituted phenols wherein the substituents do not affect coatingoperations, such as cresol and nitrophenol. Glycol ethers may also beused as capping agents. Suitable glycol ethers include ethylene glycolmonobutyl ether, diethylene glycol monobutyl ether, ethylene glycolmonomethyl ether and propylene glycol monomethyl ether.

Other suitable capping agents include oximes such as methyl ethylketoxime, acetone oxime and cyclohexanone oxime, lactams such asepsilon-caprolactam, and amines such as dibutyl amine.

The polyisocyanates can be fully blocked as described in U.S. Pat. No.3,984,299 column 1 lines 1 to 68, column 2 and column 3 lines 1 to 15,or partially blocked and reacted with the polymer backbone as describedin U.S. Pat. No. 3,947,338 column 2 lines 65 to 68, column 3 and column4 lines 1 to 30. By “blocked” is meant that the isocyanate groups havebeen reacted with a compound such that the resultant blocked isocyanategroup is stable to active hydrogens at ambient temperature but reactivewith active hydrogens in the film forming polymer at elevatedtemperatures usually between 90° C. and 200° C. In one example of thepresent invention, the polyisocyanate curing agent is a fully blockedpolyisocyanate with substantially no free isocyanate groups. By“ambient” temperature or conditions is meant without the application ofheat or other energy; for example, when a curable composition undergoesa thermosetting reaction without baking in an oven, use of forced air,irradiation, or the like to prompt the reaction, the reaction is said tooccur under ambient conditions. Usually ambient temperature ranges from60 to 90° F. (15.6 to 32.2° C.), such as a typical room temperature, 72°F. (22.2° C.).

The at least partially blocked polyisocyanate curing agent (2) can beadded to the electrodepositable composition as an individual componentor it may be added to the reaction mixture of reactants duringpreparation of the ungelled active hydrogen-containing, cationic saltgroup-containing resin (1). The at least partially blockedpolyisocyanate curing agent (2) may be present in the electrodepositablecomposition used in the processes of the present invention in an amountranging from 80 to 20 percent, often from 70 to 25, and typically from50 to 30 percent by weight, based on the total combined weight of thecationic salt group-containing resin and the curing agent.

The resinous phase of the electrodepositable coating composition furthercomprises (3) a pigment component. The pigment component comprises aninorganic, platelike pigment having an average equivalent sphericaldiameter of at least 0.2 microns and up to 5.0 microns. The averageequivalent spherical diameter may be determined using dynamic lightscattering, such as with a SEDIGRAPH III PLUS particle size analyzer,available from Micromeritics Instrument Corp. As platelike particles thepigment often has substantially opposing surfaces and particlestypically exhibit an aspect ratio of 4:1 to 10:1. Usually the inorganic,platelike pigment comprises clay and/or talc. Examples of useful claysinclude kaolin clay having an average equivalent spherical diameter ofat least 0.2 microns, such as at least 0.4 microns or at least 0.6microns, up to 5.0 microns, or up to 3.5 microns, or up to 2.5 microns,or up to 1.5 microns. Suitable talc pigments often have an averageequivalent spherical diameter of at least 0.6 microns, up 1.9 microns,or up to 1.5 microns, or up to 1.0 microns.

Typically, the inorganic platelike pigment is present in the resinousphase in an amount such that the P:B ratio in the electrodepositablecoating composition is at least 0.5 and up to 3:1, depending on thecomposition and size of the pigment. In the phrase “pigment-to-binder(or P:B) ratio”, the term “binder” refers to the total resin (1) andcuring agent (2) in the coating composition. In other words, theinorganic platelike pigment is present in the resinous phase in anamount of at least 33 percent by weight, often at least 50 percent byweight, based on the total weight of the resinous phase up to 75 percentby weight. In particular examples, talc pigments having an averageequivalent spherical diameter of 0.6 to 1.5 microns are usually used inan amount such that the P:B ratio is at least 0.5. Kaolin clays havingan average equivalent spherical diameter of 0.2 to 3.5 microns areusually used in an amount such that the P:B ratio is at least 0.5, suchas at least 1. The size (average equivalent spherical diameter), theplatelike shape, and the amount of inorganic platelike pigments (P:B)used in the curable electrodepositable coating composition allcontribute to improved corrosion barrier properties of theelectrodeposited coating composition on the metal substrate.

The pigment component comprising the inorganic platelike pigment may beadded to the resinous phase by dispersing it into either or both of thecationic salt group-containing resin (1) or the polyisocyanate curingagent (2) using conventional grinding techniques. Dispersing the pigmentcomponent in this way offers several advantages: (i) it eliminates theneed for a conventional grinding vehicle, such that the curableelectrodepositable coating composition may be essentially free of agrind vehicle, and (ii) it allows for a higher P:B ratio in theelectrodepositable coating composition. The electrodepositable coatingcomposition typically contains less than 8 percent by weight of a grindvehicle, often less than 5 percent by weight, and more often less than 3percent by weight, based on the total weight of solids in theelectrodepositable coating composition. Usually the electrodepositablecoating composition is essentially free of a grind vehicle. By“essentially free” of a material is meant that a composition has onlytrace or incidental amounts of a given material if it is present at all,and that the material is not present in an amount sufficient to affectany properties of the composition. These materials are not essential tothe composition and hence the composition is free of these materials inany appreciable or essential amount. If they are present, it is inincidental amounts only, typically less than 0.1 percent by weight,based on the total weight of solids in the composition. Additionally,corrosion barrier properties of the coating composition are improvedbecause higher crosslink densities are attained in the final curedcoating composition and costs are reduced. Usually the pigment componentis dispersed in the cationic salt group-containing resin (1) prior todispersion of the resinous phase into the aqueous medium.

The resinous phase is dispersed in an aqueous medium to prepare thecurable, electrodepositable coating composition in the form of anelectrodeposition bath. The curable, electrodepositable coatingcomposition may additionally include optional ingredients commonly usedin such compositions. For example, the composition may further comprisea hindered amine light stabilizer for UV degradation resistance. Suchhindered amine light stabilizers include those disclosed in U.S. Pat.No. 5,260,135. When they are used they are present in theelectrodepositable composition in an amount of 0.1 to 2 percent byweight, based on the total weight of resin solids in theelectrodepositable composition. Other optional additives such asadditional colorants, surfactants, further wetting agents or catalystscan be included in the composition. Catalysts suitable for use in thecurable electrodepositable composition include those known to beeffective for reactions of isocyanates with active hydrogens.

Besides water, the aqueous medium of the electrodeposition bath maycontain a coalescing solvent, surfactants, and other additives that maybe dissolved in the water. Useful coalescing solvents includehydrocarbons, alcohols, esters, ethers and ketones. The most suitablecoalescing solvents include alcohols, polyols and ketones. Specificcoalescing solvents include isopropanol, butanol, 2-ethylhexanol,isophorone, 2-methoxypentanone, ethylene and propylene glycol and themonoethyl, monobutyl and monohexyl ethers of ethylene glycol. The amountof coalescing solvent is generally between about 0.01 and 25 percent andwhen used, often from about 0.05 to about 5 percent by weight based ontotal weight of the aqueous medium.

The concentration of the resinous phase including the pigment in theaqueous medium is at least 1 and usually from 2 to 30 percent by weight,more often 10 to 30 percent by weight, based on total weight of theaqueous dispersion.

Generally, in the process of electrodeposition, the metal substratebeing coated, serving as a cathode, and an electrically conductive anodeare placed in contact with the cationic electrodepositable composition.Upon passage of an electric current between the cathode and the anodewhile they are in contact with the electrodepositable composition, anadherent film of the electrodepositable composition will deposit in asubstantially continuous manner and consistent thickness on theelectroconductive substrate.

In the method of the present invention, any of the aqueous, curableelectrodepositable coating compositions described above areelectrophoretically deposited on the substrate to form anelectrodeposited coating over at least a portion of the substrate. Thesubstrate serves as a cathode in an electrical circuit comprising thecathode and an anode, and the cathode and the anode are immersed in theaqueous electrodepositable coating composition. Electrodeposition isusually carried out at a constant voltage in the range of from about 1volt to several thousand volts, typically between 50 and 500 volts.Current density is usually between about 1.0 ampere and 15 amperes persquare foot (10.8 to 161.5 amperes per square meter) and tends todecrease quickly during the electrodeposition process, indicatingformation of a continuous self-insulating film.

After electrodeposition, the coated substrate is heated to cure thedeposited compositions. The heating or curing operation is usuallycarried out at a temperature less than 250° F. (121.1° C.), often lessthan 225° F. (107.2° C.), for a period of time sufficient to effect cureof the composition, typically ranging from 10 to 60 minutes. Thethickness of the resultant film is usually from about 10 to 50 microns.

In most conventional cationic electrodeposition bath systems, theanode(s) are comprised of a ferrous material, for example, stainlesssteel. A typical cationic bath has an acidic pH ranging from 4.0 to 7.0,and often from 5.0 to 6.0. However, in a typical electrodeposition bathsystem, the anolyte (i.e., the bath solution in the immediate area ofthe anode) can have a pH as low as 3.0 or less due to the concentrationof acid at or near the anode. At these strongly acidic pH ranges, theferrous anode can degrade, thereby releasing soluble iron into the bath.By “soluble iron” is meant Fe+2 or Fe+3 ions derived from iron saltswhich are at least partially soluble in water. During theelectrodeposition process, the soluble iron is electrodeposited alongwith the resinous binder and is present in the cured electrodepositedcoating. It has been found that the presence of iron in soluble form cancontribute to interlayer delamination of subsequently applied top coatlayers from the cured electrodeposited coating layer upon weatheringexposure. In view of the foregoing, it is desirable that theelectrodepositable coating composition of the present invention, when inthe form of an electrodeposition bath, comprises less than 10 parts permillion, typically less than 1 part per million of soluble iron. Thiscan be accomplished by the inclusion in the circuit of a non-ferrousanode.

In certain examples of the present invention, particularly when thesubstrate is an automotive body part, the coated substrate may furthercomprise a primer coating layer applied on the surface of the substratesubsequent to application and curing of the electrodepositable coatingcomposition, followed by one or more topcoats. The primer coating layerand topcoat layers may comprise any coating composition known in theart; in an automotive application, the coatings are typically curablecompositions. The coatings can comprise a resinous binder and a pigmentand/or other colorant, as well as optional additives well known in theart of coating compositions. Nonlimiting examples of resinous bindersare acrylic polymers, polyesters, alkyds, and polyurethanes.

Non-limiting examples of suitable base coat compositions includewaterborne base coats such as are disclosed in U.S. Pat. Nos. 4,403,003;4,147,679; and 5,071,904. Suitable clear coat compositions include thosedisclosed in U.S. Pat. Nos. 4,650,718; 5,814,410; 5,891,981; and WO98/14379.

The top coat compositions can be applied by conventional means includingbrushing, dipping, flow coating, spraying and the like, but they aremost often applied by spraying. The usual spray techniques and equipmentfor air spraying and electrostatic spraying and either manual orautomatic methods can be used.

After application of each top coat to the substrate, a film is formed onthe surface of the substrate by driving water out of the film by heatingor by an air-drying period. Typically, the thickness of a pigmented basecoat ranges from about 0.1 to about 5 mils (about 2.54 to about 127microns), and often about 0.4 to about 1.5 mils (about 10.16 to about38.1 microns). The thickness of a clear coat usually ranges from about0.5 to about 5 mils (about 12.7 to about 127 microns), often about 1.0to about 3 mils (about 25.4 to about 76.2 microns).

The heating will typically be only for a short period of time and willbe sufficient to ensure that any subsequently applied top coating can beapplied without any dissolution occurring at the coating interfaces.Suitable drying conditions will depend on the particular top coatcomposition and on the ambient humidity (if the top coat composition iswaterborne), but in general a drying time of from about 1 to 5 minutesat a temperature of about 80° F. to 250° F. (20° C. to 121° C.) is used.Usually between coats, the previously applied coat is flashed, that is,exposed to ambient conditions for about 1 to 20 minutes.

After application of the top coat composition(s), the coated substrateis then heated to a temperature and for a period of time sufficient toeffect cure of the coating layer(s). In the curing operation, solventsare driven off and the film-forming materials of the top coats are eachcrosslinked. The heating or curing operation is usually carried out at atemperature in the range of from 160° F. to 350° F. (71° C. to 177° C.)but if needed, lower or higher temperatures may be used as necessary toactivate crosslinking mechanisms. Cure is as defined as above.

Metal substrates coated in accordance with the method of the presentinvention demonstrate excellent corrosion resistance as determined bysalt spray and/or other cyclic corrosion resistance testing.

Each of the characteristics and examples described above, andcombinations thereof, may be said to be encompassed by the presentinvention. The present invention is thus drawn to the followingnonlimiting aspects:

1. A method of improving the corrosion resistance of a metal substratecomprising:

(a) electrophoretically depositing on the substrate a curableelectrodepositable coating composition to form an electrodepositedcoating over at least a portion of the substrate, the electrodepositablecoating composition comprising a resinous phase dispersed in an aqueousmedium, said resinous phase comprising:

(1) an ungelled active hydrogen-containing, cationic saltgroup-containing resin electrodepositable on a cathode;

(2) an at least partially blocked polyisocyanate curing agent; and

(3) a pigment component, wherein the pigment component comprises aninorganic, platelike pigment having an average equivalent sphericaldiameter of at least 0.2 microns; and wherein the inorganic platelikepigment is present in the resinous phase in an amount such that theelectrodepositable coating composition demonstrates a pigment-to-binderratio of at least 0.5; and wherein the electrodepositable coatingcomposition contains less than 8 percent by weight of a grind vehicle,based on the total weight of solids in the electrodepositable coatingcomposition; and

(b) heating the substrate to a temperature and for a time sufficient tocure the electrodeposited coating on the substrate.

2. The method of aspect 1, wherein the cationic salt group-containingresin (1) is prepared from a polyepoxide polymer, an acrylic polymer, apolyurethane polymer, and/or a polyester polymer.

3. The method of aspect 1 or 2, wherein the cationic saltgroup-containing resin (1) contains cationic amine salt groups.

4. The method of any of aspects 1 to 4, wherein the inorganic, platelikepigment comprises clay and/or talc.

5. The method of any of aspects 1 to 4, wherein the inorganic, platelikepigment comprises kaolin clay having an average equivalent sphericaldiameter of 0.2 to 5.0 microns.

6. The method of any of aspects 1 to 5, wherein the electrodepositablecoating composition demonstrates a pigment-to-binder ratio of at least1.

7. The method of aspect 5, wherein the kaolin clay has an averageequivalent spherical diameter of 0.2 to 3.5 microns.

8. The method of aspect 4, wherein the inorganic, platelike pigmentcomprises talc having an average equivalent spherical diameter of 0.6 to1.9 microns.

9. The method of any of aspects 1 to 8 wherein the curableelectrodepositable coating composition is essentially free of a grindvehicle.

10. The method of any of aspects 1 to 9 wherein the pigment component isdispersed in the cationic salt group-containing resin (1) prior todispersion of the resinous phase into the aqueous medium.

Illustrating the invention are the following examples that are not to beconsidered as limiting the invention to their details. All parts andpercentages in the examples, as well as throughout the specification,are by weight unless otherwise indicated. Example 1 is a control andrepresents the evaluation of a panel with a film containing no pigment.Example 2 is comparative and represents the evaluation of a panel with afilm prepared from an electrodepositable coating composition having apigment to binder ratio of 0.25. Example 3 represents the evaluation ofa panel with a film prepared from an electrodepositable coatingcomposition having a pigment to binder ratio of 0.5. Example 4 iscomparative and represents the evaluation of a panel with a filmprepared from an electrodepositable coating composition having a pigmentto binder ratio of 0.5, with the pigment being added to the mainfilm-forming resin as a paste using a grind vehicle present in an amountof about 8 percent by weight (U.S. Pat. No. 7,842,762 B2 (Example24(a)), at column 37, lines 1-37). Example 5 represents the evaluationof a panel with a film prepared from an electrodepositable coatingcomposition having a pigment to binder ratio of 0.75. Example 6represents the evaluation of a panel with a film prepared from anelectrodepositable coating composition having a pigment to binder ratioof 1. Example 7 represents the evaluation of a panel with a filmprepared from an electrodepositable coating composition having a pigmentto binder ratio of 1.1.

Resin Synthesis Example R1: Preparation of a Blocked PolyisocyanateCrosslinker for Electrodepositable Coating Compositions (Crosslinker I)

A blocked polyisocyanate crosslinker (Crosslinker I), suitable for usein electrodepositable coating resins, was prepared in the followingmanner: components 2-5 listed in Table 1, below, were mixed in a flaskset up for total reflux with stirring under nitrogen. The mixture washeated to a temperature of 35° C., and Component 1 was added dropwise sothat the temperature increased due to the reaction exotherm and wasmaintained under 100° C. After the addition of Component 1 was complete,a temperature of 110° C. was established in the reaction mixture and thereaction mixture held at temperature until no residual isocyanate wasdetected by IR spectroscopy. Component 6 was then added and the reactionmixture was allowed to stir for 30 minutes and cooled to ambienttemperature.

TABLE 1 Parts-by-weight Component (grams) 1. Polymeric methylenediphenyl diisocyanate ¹ 1340.00 2. Dibutyltin dilaurate 2.61 3. Methylisobutyl ketone 200.00 4. Diethylene glycol monobutyl ether 324.46 5.Ethylene glycol monobutyl ether 945.44 6. Methyl isobutyl ketone 122.85¹ Rubinate M, available from Huntsman Corporation.

Example R2: Preparation of a Cationic, Amine-Functionalized, PolyepoxideBased Resin (Resin RSP1)

A cationic, amine-functionalized, polyepoxide-based polymeric resinsuitable for use in formulating electrodepositable coating compositions,was prepared in the following manner: components 1-5 listed in Table 2,below, were combined in a flask set up for total reflux with stirringunder nitrogen. The mixture was heated to a temperature of 130° C. andallowed to exotherm (175° C. maximum). A temperature of 145° C. wasestablished in the reaction mixture and the reaction mixture was thenheld for 2 hours. Components 6-8 were then introduced into the reactionmixture and a temperature of 110° C. was established in the reactionmixture. Components 9 and 10 were then added to the reaction mixturequickly and the reaction mixture was allowed to exotherm. A temperatureof 121° C. was established in the reaction mixture and the reactionmixture held for 1 hour. After the hold, the heating source was removedfrom the reaction mixture and Component 11 was introduced slowly. Thecontent of the flask was allowed to stir while cooling to roomtemperature. The resulting Resin Synthesis Product 1 (RSP1) had a solidscontent of 65% by weight.

TABLE 2 Parts-by-weight No. Component (grams) Resin Synthesis Stage 1Bisphenol A diglycidyl ether ¹ 1659.63 2 Bisphenol A 716.64 3 BisphenolA - ethylene oxide adduct 337.50 (1/6 molar ratio BPA/EtO) 4 Methylisobutyl ketone (MIBK) 83.93 5 Ethyl triphenyl phosphonium iodide 1.62 6Bisphenol A - ethylene oxide adduct 337.50 (1/6 molar ratio BPA/EtO) 7Methyl isobutyl ketone 140.53 8 Crosslinker I ² 1943.12 9 Diethylenetriamine - MIBK diketimine ³ 153.92 10 Methyl ethanol amine 131.43 111-Methoxy-2-propanol 2117.62 ¹ EPON 828, available from HexionCorporation. ² See Example R1, above. ³ 72.7% by weight (in MIBK) of thediketimine reaction product of 1 equivalent of diethylene triamine and 2equivalents of MIBK.

Example Formulations

Formulation:

A pigment (ASP-900, kaolin available from BASF having an averageequivalent spherical diameter of 1.5 microns) was added to a resin(RSP1) and mixed under high sheer for 10 minutes, followed by additionof a catalyst paste (dibutyltin dioxide, DBTO), a dispersing agent(DYNASYLAN 4148, available from Evonik Industries) and diethylene glycolmonobutyl ether-formaldehyde adduct. High sheer mixing continued for anadditional 50 minutes. The resulting paste was dispersed in 100 g ofaqueous sulfamic acid, mixed for 20 minutes, then diluted with theremaining water.

Electrocoating:

3″×2″ ACT CRS C700 DIW panels were electrocoated at 90° F., 0.5 A andvoltages (120-400V) were adjust to achieve 0.6-0.9 mils films.

Curing:

The electrocoated panels were baked at 350° F. for 30 min.

Salt Spray:

Panels were scribed with a 1.5″ vertical line in the middle of the paneland placed in a salt spray cabinet for 1000 hours, according to ASTM B117-73 (published 1979). The corrosion creep size was measured accordingto ASTM D 1654-08 (published 2016).

Formulation Example 1 Example 2 Example 4 CONTROL COMPARATIVE Example 3COMPARATIVE Example 5 Example 6 Example 7 P/B No Pigment 0.25 0.5 0.50.75 1 1.1 Raw Materials Wt (g) Wt (g) Wt (g) Wt (g) Wt (g) Wt (g) Wt(g) Resin (RSP1) 277 246.2 205.1 163.8 176 153.8 146.2 Pigment 0 40 66.766.9 85.7 100 105 Grinding Vehicle — — — 26.5 — — — (described above)Diethylene glycol 3.6 3.2 2.7 2.7 2.3 2 1.9 monobutyl ether-formaldehyde Catalyst 7.3 6.6 5.5 4.4 4.7 4.1 3.9 DBTO* Dispersing agent0 0.8 1.3 0 1.7 2 2.1 Dispersion Sulfamic acid 4.5 4 3.4 2.7 2.9 2.5 DIWater 608 699 715.4 665 727 735.5 Total 900 1000 1000 1000 1000 1000Properties of Examples 1 to 7 Film Thickness (mils) 0.76 0.9 0.85 0.760.84 0.8 0.6 Salt Spray corrosion 1.04 0.93 0.83 1.9 0.84 0.86 0.92creep size (mm)** *DBTO added as a dispersion; prepared as described inU.S. Pat. No. 7,070,683 B2, at column 16, lines 10-30 **The panels weretested after 1000 hours of salt spray exposure, except for Example 1(control), which was tested after 850 hours of salt spray exposure.

Whereas particular examples of this invention have been described abovefor purposes of illustration, it will be evident to those skilled in theart that numerous variations of the details of the present invention maybe made without departing from the invention as defined in the appendedclaims. It is understood, therefore, that this invention is not limitedto the particular examples disclosed, but it is intended to covermodifications which are within the spirit and scope of the invention, asdefined by the appended claims.

Therefore, we claim:
 1. A method of improving the corrosion resistanceof a metal substrate comprising: (a) electrophoretically depositing onthe substrate a curable electrodepositable coating composition to forman electrodeposited coating over at least a portion of the substrate,the electrodepositable coating composition comprising a resinous phasedispersed in an aqueous medium, said resinous phase comprising: (1) anungelled active hydrogen-containing, cationic salt group-containingresin electrodepositable on a cathode; (2) an at least partially blockedpolyisocyanate curing agent; and (3) a pigment component, wherein thepigment component comprises an inorganic, platelike pigment having anaverage equivalent spherical diameter of at least 0.2 microns; andwherein the inorganic platelike pigment is present in the resinous phasein an amount such that the electrodepositable coating compositiondemonstrates a pigment-to-binder ratio of at least 1; and wherein theelectrodepositable coating composition contains less than 8 percent byweight of a grind vehicle, based on the total weight of solids in theelectrodepositable coating composition; and (b) heating the substrate toa temperature and for a time sufficient to cure the electrodepositedcoating on the substrate.
 2. The method of claim 1, wherein the cationicsalt group-containing resin (1) is prepared from a polyepoxide polymer,an acrylic polymer, a polyurethane polymer, and/or a polyester polymer.3. The method of claim 1, wherein the cationic salt group-containingresin (1) contains cationic amine salt groups.
 4. The method of claim 1,wherein the inorganic, platelike pigment comprises clay.
 5. The methodof claim 4, wherein the inorganic, platelike pigment comprises kaolinclay having an average equivalent spherical diameter of 0.2 to 5.0microns.
 6. The method of claim 5, wherein the kaolin clay has anaverage equivalent spherical diameter of 0.2 to 3.5 microns.
 7. Themethod of claim 1 wherein the curable electrodepositable coatingcomposition is essentially free of a grind vehicle.
 8. The method ofclaim 7 wherein the pigment component is dispersed in the cationic saltgroup-containing resin (1) prior to dispersion of the resinous phaseinto the aqueous medium.